I have not used the tamron 17-50, but I did just buy the canon 50mm 1.4, and LOVE IT! Very fast focus, the build quality is pretty good, the bokeh is fantastic and smooth, and the extra light is indispensable in some situations. I just posted some pictures I took in a bar; It was so dark that I was shooting ISO 1600, and between 1.4 and 2.0 the entire time. It wont give you as good of an Idea for fashion photograph, but its all that I have uploaded right now.

yes there is! i have tamron 17-50 and 28-75, both f2.8, but 50mm 1.4 is a totally different thing!

Firstly its faster.
Secondly it has wider apperature and that meens more light onto your sensor and a lot more shallow depth of field.
Finallly: mostly fixed lenses like 28mm, 50mm, etc gives more decent and produces better image quality than zooms!